This invention relates to an optoelectronics system and method for measuring one or more properties of an environment. In particular the invention relates to a system for measuring properties such as pH, PCO.sub.2 and PO.sub.2 of an environment by alternately transmitting light of two different wavelengths to the sensor and computing the value of the environmental property from the ratio of the two return light signals from the sensor. The invention is particularly useful for monitoring pH, PCO.sub.2 and PO.sub.2 in medical applications.
Various fiber optic chemical sensors have been developed recently, and particularly for use in medical applications. Fiber optic chemical sensors are based on the principle that the absorbance or fluorescence of a particular material changes with a particular property to be measured. For example, certain fiber optic pH sensors are based on the principle that the absorbance of the sensing material changes with pH of the surrounding environment. It has been found that a colorimetric pH indicating dye, such as phenol red, when bound to a polyacrylamide hydrogel by copolymerization of the dye with the acrylamide monomer produces a pH sensing material. The dye in the sensing material changes color with changes in pH and the color change corresponds to a change in the amount of light of a specified wavelength absorbed by the dye (e.g. green light in the case of phenol red). A fiber optic pH sensing probe consists of a pH sensing material bound to one or more optical waveguides. Light at a particular wavelength is transmitted down a waveguide to the pH sensor where the sensing material attenuates the light with respect to a second wavelength. This attenuated light is reflected or transmitted back down the same or another optical waveguide to a detector.
By changing the sensing element at the tip of the fiber optic probe, different properties of the environment can be measured. For example, phenol red bound to a polyacrylamide hydrogel can be used as a pH sensing medium; the addition of a bicarbonate solution to the pH sensing material produces a PCO.sub.2 sensor, and the choice of an oxygen sensing medium such as a fluorescent dye chosen from the group of pyrene derivatives and perylene derivatives produces a PO.sub.2 sensing sensor.
A pH sensing fiber optic probe is described in U.S. Pat. No. 4,200,110 to Peterson et al. Another fiber optic pH sensor is described in co-owned, co-pending application Ser. No. 747,488 filed June 21, 1985 titled "Fiber Optic pH Sensor Having Low Drift Rate", which is incorporated herein by reference. A fiber optic CO sensor is described in co-owned, co-pending application Ser. No. 877,572 filed June 23, 1986 titled "Fiber Optic CO.sub.2 Sensor", which is incorporated herein by reference. Fiber optic sensors for measuring the partial pressure of oxygen are described in co-owned, co-pending application Ser. No. 698,282 filed Feb. 4, 1985 titled "Single Optical Fiber For Measuring the Partial Pressure of Oxygen" and Ser. No. 699,515 filed Feb. 7, 1985 titled "Optical Sensor for Monitoring the Partial Pressure of Oxygen", both of which are incorporated herein by reference.
Although fiber optic systems have been devised using light of a single wavelength, to achieve greater accuracy in measuring the particular chemical property of an environment, two different wavelengths of light are transmitted to the sensing material and the ratio of return light signals is computed. U.S. Pat. No. 3,799,672 to Vurek describes a device for monitoring blood oxygen saturation in which infrared radiation and visible red light are alternately shined through blood as it flows through a plastic cuvet. The ratio of the infrared signal to the red signal received by a phototransistor is computed by an electrical feedback system, the ratio being linearly related to blood oxygen saturation. Vurek uses two separate light sources to provide the two different wavelengths. Vurek recognized that the use of two separate light sources could introduce unreliability into the system due to drift, thermal changes and component aging. To eliminate some of these problems Vurek chose similar light sources for both light signals. In this case Vurek chose infrared and red LEDs. Vurek recognized that an advantage could be achieved when the signal for the denominator term in the ratio is maintained at a high level so that a good signal-to-noise ratio is maintained even at low saturation levels. To achieve this condition Vurek introduces a control or feedback system which alternately energizes the light sources such that the output of the second light source is controlled by and proportional to the optical signal of the first light source.
A major disadvantage of this scheme is that Vurek relies heavily on the fact that the LED output light power is directly and linearly related to input current. Vurek's system does not compensate for light sources whose output light is not directly and linearly related to input current, nor to light sources whose output light power or wavelength may change over time due to thermal effects and component aging.
U.S. Pat. No. 4,476,870 to Peterson describes a fiber optic oxygen sensing probe and system for measuring oxygen partial pressure in blood or tissue. Like Vurek, Peterson measures the ratio of the intensities of light of two different wavelengths to determine oxygen partial pressure. To minimize some of the problems associated with changes in light output from two light sources, Peterson uses a single light source which transmits light alternately through a blue filter and a green filter. In this way Peterson compensates for variations in light that are linearly related since they will be cancelled out in the ratio. However, Peterson's system does not compensate for changes in spectral output of the light source over time due to drift, temperature instability, etc., which are non-linear.
Therefore it is an object of the present invention to provide an optoelectronic system and method to measure chemical properties of an environment which is stable with respect to time, component aging and temperature variations.
It is another object of the present invention to provide an optoelectronic system and method which will measure the pH, PCO.sub.2 and P0.sub.2 of an environment.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.